By Jin A, Darzi AJ, Dargham A, Liddar N, Bozorgi S, Sohrevardi S, Zhang M, Torabiardakani K, Couban RJ, Khalili M, Busse JW, Sadeghirad B.

Background: Increasing legalization of recreational cannabis and availability of cannabinoid products has resulted in expanded use, which is associated with adverse effects including concerns over increased risk of motor vehicle collision (MVC). We aimed to explore the association between cannabis consumption and MVC.

Methods: We searched MEDLINE, EMBASE, CINAHL, Cochrane library, SCOPUS, PsycInfo, Web of Science, TRID from inception to November 2024. We included studies assessing the association between cannabis consumption on MVC fatalities, any injuries, and culpability/unsafe driving actions. Pairs of reviewers independently screened search results, extracted data, and assessed risk of bias. We used a DerSimonian and Laird random-effects model for all meta-analyses and the GRADE approach to assess the certainty of evidence.

Results: We included 31 studies with 328,388 individuals. Low certainty evidence suggests that cannabis consumption may be associated with an increased risk of MVC fatality (8 studies, OR 1.55, 95% CI: 1.20 to 1.98) with an absolute risk increase (ARI) of 14 more deaths per 100,000 MVC's. Low certainty evidence from 9 case-control studies suggests cannabis consumption may be associated with an increased risk of injury due to MVC (OR 2.00, [95% CI: 1.31-3.07]; absolute risk increase of 6.8%). We are uncertain about the association of cannabis consumption with MVC culpability/unsafe driving action as the evidence was only very low certainty.

Conclusions: Low certainty evidence suggests that cannabis consumption may increase risk of MVC fatality and risk of injury from MVC. The association between cannabis use and risk of unsafe driving is uncertain.

Int J Drug Policy. 2025 Aug;142:104832. doi: 10.1016/j.drugpo.2025.104832. Epub 2025 May 13. PMID: 40367728.

By B.C.Tefft, and L.S.. & Arnold,

Washington State Initiative 502 (I-502), effective Dec. 6, 2012, legalized possession of small amounts of cannabis for recreational use by adults aged 21 years and older. It also included a prohibition against driving with 5 or more nanograms of delta-9-tetrahydrocannabinol (THC) per milliliter of blood, along with a zero tolerance prohibition for drivers younger than 21 years of age. THC is the main psychoactive component in cannabis and detection of THC in blood is suggestive of recent use. A previous study by the AAA Foundation for Traffic Safety examined data from drivers involved in fatal crashes in Washington State in years 2010-2014 and estimated that the proportion of drivers with detectable THC approximately doubled several months after I-502 became effective (Tefft et al., 2016). The research reported here updates the previous study with three additional years of data, post-legalization. Multiple imputation was used to estimate the proportion of drivers who were THC-positive among those who were not tested for drugs or whose test results were unavailable. Results indicate that five years after I-502, the proportion of fatal-crash-involved drivers who are THC-positive has remained approximately double the level observed before I-502. An estimated 21% of all drivers involved in fatal crashes in Washington State in 2017 were THC-positive, higher than in any other year in the 10-year period examined

Washington, D.C.: AAA Foundation for Traffic Safety , 2020. 6p.